1. Field of the Invention
The present invention relates to a driving apparatus for intake and exhaust valves of an internal combustion engine of, for instance, a vehicle, and particularly to a driving apparatus which can control variably the rotational phase and the lift of valve of the engine according to the engine operating conditions.
2. Description of the Prior Art
An example of the apparatus which controls the open and close timing and the lift of the intake and exhaust valve is disclosed in U.S. Pat. No. 3,413,965. The apparatus includes an rocker arm, one end of which contacts a valve driving cam and the other end being coupled with the stem end of an intake or exhaust valve so that the rocker arm may be freely pivotal around the point of coupling. The back surface which is curved of the rocker arm contacts a lever. According to the rotational movement of the valve driving cam, the rocker arm provides rocking motion with the point of contact between the rocker arm and the lever as a fulcrum to drive the valve. Cne end of the lever is pivotally supported by an engine body, and the pivotal movement (slant) of the lever is controlled by a valve lift controlling cam which is arranged to contact the other end of the lever. The valve lift controlling cam is turned by means of a hydraulic actuator, etc., according to engine operating conditions so that the lift characteristic of valve may variably controlled.
In the above prior art example, if the valve open timing is delayed in the slow-speed and small-load operating conditions in comparison with the high-speed and large-load conditions of the engine, the valve close timing will simultaneously be advanced. As a result, the closure timing of intake valve comes close to the bottom dead center in an intake stroke, if the valve open timing is delayed in the slow-speed and small-load operating conditions to reduce the overlap between the intake and exhaust valves, reduce the residual gas in the combustion chamber and improve the combustion conditions. Thus, the pumping loss may not be decreased, and the thermal efficiency may not be improved.
Another example of the valve driving apparatus which controls the rotational phase of a valve driving cam is disclosed in Japanese Patent Application Publication No. 50-76428. The apparatus includes a plurality of weights arranged on a cam gear which is fixed to a cam shaft to rotate a cam for driving a valve. The weights are pivotally fixed on the cam gear and moved due to the centrifugal force applied on the weights, if the cam gear is rotated. The pivotal movement of the weights changes the rotational phase of the cam.
In the above prior art example, the valve open and close timing is advanced or delayed in response to engine speed under the constant lift of valve. As a result, if the intake valve close timing is advanced in the slow-speed operating state of engine to bring the timing close to the bottom dead center to improve the changing efficiency, the intake valve open timing comes close to the top dead center so that the counter flow of exhaust gas may be increased during the overlap period of the intake and exhaust valves. Thus, the charging efficiency will not be improved. In the slow-speed operating state, the combustion conditions will be deteriorated due to the counter flow of exhaust gas so that the stable low-speed operability will not be secured.